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Abstract:

A method of forming ceramic pattern structures of silicon carbide film
includes depositing an electron-beam resist or a photo-resist onto a
substrate. A portion of the resist is selectively removed from the
substrate to form a resist pattern on the substrate. A film of
pre-ceramic polymer that includes silicon and carbon is deposited onto
the substrate and resist pattern and the pre-ceramic polymer film is
cured. A portion of the cured pre-ceramic polymer film on the resist
pattern is removed, thereby forming a pre-ceramic polymer pattern on the
substrate. The pre-ceramic polymer pattern is then converted to a ceramic
pattern.

Claims:

1. A method of forming a ceramic pattern, comprising the steps of: a)
depositing an electron-beam resist or a photo-resist onto a substrate; b)
selectively exposing the resist to an electron beam or radiation to
thereby degrade, volatilize, polymerize, crosslink, or otherwise alter
the solubility of a portion of the resist; c) removing only the exposed
or only the unexposed portion of the resist from the substrate to form a
resist pattern on the substrate; d) depositing a pre-ceramic polymer that
includes silicon and carbon onto the substrate and resist pattern; e)
removing a portion of the pre-ceramic polymer previously deposited on the
resist pattern, thereby forming a pre-ceramic polymer pattern on the
substrate; and f) converting the pre-ceramic polymer pattern to a ceramic
pattern.

2. The method of claim 1, wherein the resist and/or the pre-ceramic
polymer is deposited onto the substrate by spin-coating.

3. The method of claim 1, wherein the electron-beam resist is poly(methyl
methacrylate).

4. The method of claim 1, wherein the photo-resist includes at least one
member selected from the group consisting of diazonaphthaquinone (DNQ)
and polymethyl glutarimide (PMGI).

5. The method of claim 4, wherein the photo-resist includes DNQ, and
further includes a phenol formaldehyde resin.

6. The method of claim 1, wherein the pre-ceramic polymer is cured prior
to resist pattern removal, and the resist pattern is removed using a
solvent that does not substantially degrade the cured pre-ceramic
polymer.

7. The method of claim 1, wherein the thickness of the resist on the
substrate is in a range of between about 1 nanometer and 5 micrometers.

8. The method of claim 1, wherein the pre-ceramic polymer includes at
least one polymer having a ratio of silicon-to-carbon of about 1:1 and a
backbone structure where the majority of bonds are Si--C.

9. The method of claim 1, wherein the pre-ceramic polymer pattern has an
average thickness in a range of between about 1 nanometer and 5
micrometers.

10. The method of claim 1, wherein dopant atoms are inserted into the
pre-ceramic polymer.

11. The method of claim 1, wherein the substrate, resist layer and
pre-ceramic polymer are heated in an inert atmosphere, or one containing
a reactive gas that effects doping of the resulting ceramic, during the
step of converting the pre-ceramic polymer to a ceramic.

12. A method of forming a ceramic structure, comprising the steps of: a)
preparing a mold comprising cavities having the shape of a desired
structure; b) introducing a pre-ceramic polymer into the mold; c) curing
the pre-ceramic polymer; d) removing the mold from the pre-ceramic
polymer, thereby forming a pre-ceramic polymer structure; e) converting
the pre-ceramic polymer structure to a ceramic structure.

13. The method of claim 12, wherein the pre-ceramic polymer includes at
least one polymer having a ratio of silicon-to-carbon of about 1:1 and a
backbone structure where the majority of bonds are Si--C.

14. The method of claim 12, wherein the pre-ceramic polymer structure has
an average thickness in a range of between about 1 nanometer and 5
micrometers.

15. The method of claim 12, wherein the mold is placed against a surface
of a substrate on which the desired structure is to be formed.

16. A patterned structure formed by the method of claim 1.

17. A patterned structure formed by the method of claim 12.

Description:

RELATED APPLICATION

[0001] This application is a continuation-in-part of International
Application No. PCT/US2010/058575, filed on Dec. 1, 2010, published in
English, which claims the benefit of U.S. Provisional Application No.
61/265,582, filed on Dec. 1, 2009. The entire teachings of the above
applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0003] Silicon Carbide (SiC) has been a material of interest for
microelectronics for a considerable time. The large bandgap and chemical
stability enables it to operate in environments where silicon can not
function. Silicon electronics cease to function properly if the operating
temperature exceeds 350° C. whereas SiC based devices have been
shown to operate at temperatures as high as 650° C. The chemical
inertness of SiC has generated interest in applications for chemical and
biological sensors. Current uses for SiC based electronics largely focus
on power applications where the high dielectric breakdown strength and
temperature characteristics make it stand out. Applications in non-power
applications have been sporadic with the most notable being their use in
high frequency transistors, resulting in higher mobility and higher power
density, for military applications and as the original blue LED, nearly a
decade before indium nitride came on the scene.

[0004] Many advanced microelectronic devices require the removal of
surrounding materials from the planar substrate surface. Field effect
transistors based on the wrap around gate design require the channel to
be exposed to the gate not only on the top but also on the sides, leading
to lower off state leakage current. For example, Micro- and Nano-Electro
Mechanical Systems (MEMS and NEMS respectively) require structures that
are detached from the substrate to form moving mechanical components. In
addition, SiC has promising optical properties in the mid infrared band
and could potentially serve as a waveguide in that frequency range. These
applications all require the ability to form structures with well defined
edges.

[0005] One of the impediments to the wider use of SiC is the difficulties
involved in etching to produce devices. The most common wet etching
technique uses molten potassium hydroxide, at a temperature of
˜400° C. This is highly undesirable for several reasons. The
presence of potassium would require stringent isolation from any silicon
process line. Potassium, along with the other alkali metals are well know
for diffusing into silicon and creating what are known as deep level
charge traps. These traps degrade the performance of any advanced digital
circuit. In addition, the etching process requires a mask that is
chemically resistant to the etchant yet be easily removed after the etch.
In the case of molten potassium hydroxide, the choice of mask material is
reduced to the noble metals or carbon, none of which makes for a
practical material. The chemical stability of SiC has precluded any
reasonable wet etching option, leaving the more expensive options of dry
etching via reactive ion etching and inductively coupled plasma etching.

SUMMARY OF THE INVENTION

[0006] The invention is directed to a method of forming a patterned
ceramic structures. According to one method, an electron-beam resist or a
photo-resist is deposited onto a substrate. The resist is selectively
exposed to an electron beam or radiation to thereby degrade, volatilize,
polymerize, crosslink, or otherwise alter the solubility of a portion of
the resist. Then, only the exposed or only the unexposed portion of the
resist is removed from the substrate to form a resist pattern on the
substrate. A structure, such as a film, of pre-ceramic polymer that
includes silicon and carbon is deposited onto the substrate and resist
pattern and the pre-ceramic polymer film of structure is cured. The
resist pattern and a portion of the cured pre-ceramic polymer film or
structure on the resist pattern are removed, thereby forming a
pre-ceramic polymer pattern on the substrate. Finally, the pre-ceramic
polymer pattern is converted to a ceramic pattern.

[0007] In another aspect, the invention is directed to a method of forming
a patterned ceramic structures, such as a film, that includes preparing a
mold comprising cavities having the shape of a desired structure. The
mold is placed against a surface of a substrate on which the desired
structure is to be formed. A pre-ceramic polymer is introduced into the
mold. The pre-ceramic polymer is then cured and the mold is removed from
the substrate, thereby forming a pre-ceramic polymer pattern on the
substrate. Finally, the pre-ceramic polymer pattern is converted to a
ceramic pattern.

[0008] The invention also includes patterned ceramic structures formed by
the methods of the invention.

[0009] For SiC to gain wider acceptance, the processing technology
utilized should be as compatible with what is used for silicon as
possible. We have developed a method for creating patterned structures,
such as thin films, of SiC using a process that involves lithography and
thermal treatment. The chemicals used do not present any additional risk
to silicon process lines for introduction of contaminants. The patterning
is able to use the exact same exposure systems as are currently used.
Finally, the thermal processing for forming the structures is well within
the temperature range used in silicon wafer processing. Fundamentally,
the key advantage to these features is that adoption of this technique
can capitalize on the immense expenditures on the development of the
silicon process technologies. It also does not need the construction of
isolated processing facilities to avoid contamination of silicon lines.

[0010] This invention has many advantages. For example, very thin films,
wherein the silicon carbide layer is essentially transparent, can be
formed. This is indicative of very high purity semiconductor grade
silicon carbide. Furthermore, features as small as 50 nanometers can be
formed. The silicon carbide structure formed by the method of the
invention can be employed to fabricate nanowire transistors, electrical
components having chemical and biological sensor applications and field
effect transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawings will be provided by the Office upon
request and payment of the necessary fee.

[0012] The foregoing will be apparent from the following more particular
description of example embodiments of the invention, as illustrated in
the accompanying drawings in which like reference characters refer to the
same parts throughout the different views. The drawings are not
necessarily to scale, emphasis instead being placed upon illustrating
embodiments of the present invention.

[0013] FIG. 1 is a SEM image of patterned SiC according to the present
invention

[0014] FIG. 2 is a SEM micrograph of SiC nanowires formed by polymer
molding of PMMA patterned by e-beam lithography;

[0015] FIG. 3 is an AFM image of SiC nano-disks 100 nm in diameter and 4
nm thick formed by polymer molding of PMMA patterned by e-beam
lithography;

[0016] FIG. 4 is a graph of current vs. voltage curves for undoped SiC
films, with and without illumination with visible light.

[0017] FIG. 5 is a graph of a current-voltage curve for a Si--SiC
heterojunction diode.

[0018] FIG. 6 is a graph of a current-voltage curve for a Si--SiC
heterojunction diode.

DETAILED DESCRIPTION OF THE INVENTION

[0019] A description of example embodiments of the invention follows. The
teachings of all patents, published applications and references cited
herein are incorporated by reference in their entirety.

[0020] In one aspect, the invention is directed to methods of producing
patterned structures, such as films, of silicon carbide, such as films of
silicon carbide. Patterned structures can be made with feature sizes down
to 50 nm. The methods for making patterned silicon carbide in accordance
with the present invention include the formation of the desired pattern
in the pre-ceramic polymer first, and then subsequent conversion to SiC.

[0021] In one embodiment, the method includes depositing an electron beam
resist or a photo-resist onto a substrate, and selectively exposing the
resist to an electron beam or radiation to thereby degrade, volatilize,
polymerize, crosslink, or otherwise alter the solubility of a portion of
the resist. An electron-beam resist, such as poly(methylmethacrylate)
(PMMA), or a suitable photo-resist, such as a blend of
diazonaphthaquinone (DNQ) and Novolac (a phenol formaldehyde resin), or
poly methyl glutarimide (PMGI) is deposited in a film onto a substrate. A
"photo-resist," as that term is employed herein, means a film-forming
solid that is sensitive to electromagnetic radiation, such as
ultraviolet, infrared, x-ray and visible radiation, either alone or in
the presence of sensitizers, with exposure to such radiation causing the
resist to degrade, volatilize, polymerize, crosslink, or otherwise alter
its solubility in those areas exposed.

[0022] Preferably, the electron-beam resist or photo-resist is dissolved
or dispersed in a suitable liquid medium, such as a solvent (e.g.,
anisole). The liquid medium is allowed to evaporate, thereby leaving a
resist film on the surface of the substrate. The uniformity of the film
is achieved by selecting a liquid medium having a desired evaporation
rate in order to maintain a desired thickness and uniformity of the
resist layer.

[0023] Areas of the resist are then selectively exposed to an electron
beam or radiation to thereby degrade, volatilize, polymerize, crosslink,
or otherwise alter the solubility of a portion of the resist. In the case
where the resist is degraded or becomes more soluble due to electron beam
or radiation exposure ("positive tone"), the exposed portion is removed
from the substrate to form a pattern of resist on the substrate. In the
case where the resist is further polymerized/crosslinked or becomes less
soluble due to electron beam or radiation exposure ("negative tone"), the
unexposed portion is removed from the substrate to form a pattern of
resist on the substrate. After exposure, a resist pattern is formed on
the substrate by removing either only the exposed portion or unexposed
portion of the resist.

[0024] A film or structure of pre-ceramic polymer that includes silicon
and carbon is then deposited, for example, by spin coating, onto the
substrate and resist pattern. The pre-ceramic polymer may be diluted in a
liquid medium to facilitate deposition. In the case of spin coating, the
uniformity of the resulting pre-ceramic polymer depends on the viscosity
of the pre-ceramic polymer and the evaporation rate of the liquid medium.
Alternatively, the pre-ceramic is deposited by placing the substrate
under a solvent thinned polymer solution with a known polymer
concentration and specific liquid height above the substrate. Evaporation
of the liquid medium leads to a uniform film deposited on the substrate.
As with the application of the resist film, the uniformity of the
pre-ceramic polymer is achieved by selecting a solvent having a desired
evaporation rate. For example, xylene, having an evaporation rate of 0.6
(based on a baseline of butyl acetate=1) and cyclohexane, having an
evaporation rate of 5.5 have been found to produced films of desired
uniformity.

[0025] Certain SiC precursor polymers having an exact 1:1 molar ratio of
silicon to carbon in particular are especially well-suited to produce
semiconductor-grade SiC. While numerous SiC precursors exist, it is very
rare to find materials containing an exact 1:1 molar ratio of silicon to
carbon. This is problematic because any deviation from a 1:1
stoichiometry results in a substantial defect concentration, most
commonly producing the familiar black appearance of silicon carbide most
are used to (due to excess carbon) and making the material effectively
unusable as a semiconductor.

[0026] One suitable SiC precursor polymer Starfire SMP-10 (Starfire
Systems), is primarily poly(silaethylene), a.k.a.
poly(silylenemethylene), which is effectively identical to polyethylene
but with every other carbon replaced with a silicon atom. This material
has two distinct advantages; first, it contains a 1:1 molar ratio of
silicon to carbon. Second, all bonds within a linear
poly(silylenemethylene) are silicon-carbon bonds. This is important
because even with perfect stoichiometry, the possibility to form defects
does not disappear; if atoms of the same type cluster, this again results
in defects that may critically degrade electronic performance. In this
case, there is no need to break and reform bonds to arrive at the desired
result of having every silicon atom bonded only to carbon atoms and
vice-versa. This is in contrast to a polymers like
poly(hydromethylsilane), a polymer that shares with
poly(silylenemethylene) a 1:1 molar ratio of silicon to carbon but whose
backbone contains only Si--Si bonds, all of which must be broken in order
to form pure, defect-free silicon carbide. Thus, a silicon carbide
precursor having a 1:1 molar ratio of silicon to carbon and having a
backbone structure where the majority of bonds present are Si--C is
recommended.

[0027] The pre-ceramic polymer used is primarily poly(silaethylene),
a.k.a. poly(silylene methylene), and is available as SMP-10 from Starfire
Technologies (Ceram. Trans. 144 87 (2002)). The composition and structure
of linear chains of this type of polycarbosilane (described by the
formula --[SiH2CH2]n--) strongly favor the formation of
stoichiometric silicon carbide, rather than silicon- or carbon-rich
materials whose electronic properties would be of limited utility
(Struct. Bond. 101 59 (2002)). For the same reasons this material
represents an especially efficient precursor, with quoted ceramic yields
of 75-82%, nearly as high as the theoretical yield of ˜91% based on
composition alone. While in its neat form this polymer is too viscous to
achieve a uniform structure by spin coating, dilution with an appropriate
liquid medium was identified as a useful strategy for spin-coating this
material. The selection of the liquid medium was dependent on several
factors. Most importantly, the liquid medium should not dissolve the
resist, or the patterns made during lithography will be damaged. Second,
it should be sufficiently volatile to produce a more or less liquid
medium-free film following coating, but without being so volatile as to
hinder film formation through rapid viscosity increases during spinning.
It was also important to consider a liquid medium that would not
chemically alter the polymer, nor leave any residue behind that might
contaminate the converted ceramic--in particular, by increasing the
carbon content. Dissolving the SMP-10 in cyclohexane at a 4:1 ratio of
liquid medium to polymer or greater generally yields uniform structures
sufficiently free of liquid medium to produce transparent SiC upon
pyrolysis.

[0028] The ability of the pre-ceramic polymer to adhere to the silicon
surface following deposition and conversion highlights another advantage
of this technique. It is known that Si--H groups in this polymer will, in
the presence of water, slowly oxidize to produce Si--OH groups, which may
then react with one another to create siloxane crosslinks. Likewise,
several dehydrogenative coupling reactions are also possible, including
Si--H reacting with Si--OH or C--H bonds to form siloxane or carbosilane
linkages. As Si--OH and/or Si--H bonds are generally present on the
surface of silicon wafers (with exact type and number dependent on
process history), the ability of the pre-ceramic polymer to form covalent
bonds with the substrate is evident. In our case in particular, the
modified RCA cleaning process employed ensures that a large concentration
of Si--OH groups is present on the surface, favoring good adhesion.

[0029] Dopant atoms may be inserted into pre-ceramic polymer material
while it is still in its polymeric form. Of particular interest are other
pre-ceramic polymers/liquid precursors containing atoms, the majority of
which are electron rich vs. SiC (n-type) or electron-poor vs. SiC
(p-type) and whose crosslinking and subsequent thermal conversion
(typically under inert atmosphere) lead to a doped semiconductor (as
opposed to oxygen, for instance, which, while electron rich vs. SiC, will
lead to the formation of an insulator). Polysilazanes are excellent
n-type dopants, given their established utility as precursors for silicon
nitride. The addition of a small amount of polysilazane allows formation
of N-doped (n-type) SiC with minimal addition of excess carbon, which is
always a concern from the point of view of electronic properties. In
particular polysilazanes have been identified which are not only miscible
with standard SiC precursor polymers at the needed doping levels, but
which will react with the SiC precursor polymers on the molecular level,
producing a homogeneous pre-ceramic polymer network with the dopant atoms
"built-in". Such a mixture may then be processed in an identical fashion
to the pure SiC precursor polymer, making this an exceptionally
convenient approach to doping. In addition to this, more traditional
techniques may also be used; for instance, while boranes and other
oxygen-free liquid organoboron compounds tend to be pyrophoric/unstable,
making the aforementioned approach less attractive (though still
possible), the ion-implantation of boron (B) and aluminum (Al) into SiC
films has been demonstrated in the research literature and represents a
straightforward means of achieving p-type doping (see J. Electron. Mater.
25 879-884 (1996), for example, the teachings of which are incorporated
herein by reference). Another possibility is inclusion of elemental
compounds directly; for instance, in the case of p-type dopants, boron
nanopowder or liquid gallium metal may be physically mixed into the SiC
precursor polymer prior to conversion. A third approach to doping in
general involves the use of gases containing only the dopant atom in
question and fugitive atoms that are readily removed via heat treatment
(hydrogen, for instance). In this scenario, the precursor polymer might
be exposed to an atmosphere containing some amount of diborane, dialane,
digallane, ammonia, phosphine or arsine vapor, to give several examples,
prior to and/or during the conversion process, with the reactive nature
of these gases causing the inclusion of the desired dopants (B, Al, Ga,
N, P or Ar in the examples given) into the precursor polymer.

[0030] After application of the pre-ceramic polymer, a portion of the
polymer is removed thereby forming a pre-ceramic polymer pattern on the
substrate. A suitable solvent for dissolving the resist pattern without
substantially attacking the pre-ceramic polymer is introduced. As the
resist is washed away, the cured pre-ceramic polymer sitting atop the
resist will lose its only means of support and will be removed as well,
leaving behind the cured pre-ceramic polymer that is directly bonded to
the substrate.

[0031] Photoresists may contain non-volatile materials which would result
in the deposition of ash or other unwanted substances on the substrate.
If a resist is used that produces such interfering residue as a result of
thermal degradation, the pre-ceramic polymer is first cured, and the
resist is removed with a suitable solvent, such as acetone or a resist
stripper, such as N-methyl-2-pyrrolidone, that is specifically selected
to be aggressive towards the remaining resist while leaving the cured
pre-ceramic polymer phase adhered and substantially intact.

[0032] The pre-ceramic polymer pattern is next converted to a ceramic
pattern, for example, by placing the substrate into a furnace filled with
inert gas and heat to a specified temperature at which the polymer
undergoes a chemical conversion to the "ceramic" form. Any remaining
resist is depolymerized or otherwise decomposed to volatile species
without the production of residues detrimental to the subsequent
applications of the material. While this is typically performed in an
inert atmosphere in order to ensure that no elements other than those
already present in the cured pre-ceramic polymer are incorporated into a
final ceramic, it should be recognized that further modifications to the
composition and properties of the final ceramic may be readily achieved
through the intentional introduction of reactive species into the
atmosphere of the furnace either throughout the process or at specific
points during the process, as noted previously.

[0033] Any pre-ceramic polymer not in contact with the substrate is lifted
off, leaving behind a pattern of SiC (FIG. 1). During the conversion
process, the remaining electron-beam resist or photo-resist volatilizes
without leaving residue on the substrate. The end result is a film of the
SiC wherever the pattern was made in the PMMA. By this method,
electron-beam or photo-lithography structures can be made with feature
sizes down to 50 nm.

[0034] The reported technique has been demonstrated to have the capability
to form structures with well defined edges and feature sizes comparable
to the current generation of commercial microelectronic devices. The
limitations on structure size are on the electron beam exposure system.
There is no indication from the analysis that the reported resolution
represents the ultimate limit for the resolution of the SiC patterning.
The polymer employed contains no alkali metals, nor any other element
that would be considered a contaminant for silicon processing. The
fabrication process employs, photoresist patterning via electron beam-
and photo-lithography, spin coating, and thermal treatment in an inert
atmosphere. Each of these steps is an integral part of the production of
any microelectronic device, thus guaranteeing compatibility with silicon
foundries.

[0035] Patterning the SiC films by the method of the invention avoids the
significant issues that are normally faced when processing SiC.
Typically, if the SiC needs to be etched to form structures, it is either
done with molten potassium hydroxide at ˜400° C. or via
reactive ion etching. The former is a difficult material to handle and
requires conditions far beyond what the semiconductor industry employs
for silicon etching. Reactive ion etching is common in industry, but it
is expensive and is used as minimally as possible. The method of the
invention can readily be implemented into a standard silicon fabrication
line; there are no chemicals, temperatures, or special processing
requirements that exceed what is in common use today.

[0036] Alternatively, patterned structures can also be formed by preparing
a mold comprising cavities having the shape of a desired structure. The
mold is placed against a surface of a substrate on which the desired
structure is to be formed. A pre-ceramic polymer is introduced into the
mold. The pre-ceramic polymer is then cured and the mold is removed from
the substrate, thereby forming a pre-ceramic polymer pattern on the
substrate. Finally, the pre-ceramic polymer pattern is converted to a
ceramic pattern. Any of the standard tooling used in polymer molding,
such as metal, silicon, and/or heat resistant polymers, can be used to
form the mold. For example, a patterned silicone rubber stamp can be used
to stamp the liquid pre-ceramic polymer onto a substrate. In another
embodiment, the stamp is pressed onto a substrate coated with a film of
the liquid pre-ceramic polymer, thereby displacing the polymer out of the
areas of contact between the stamp and the substrate and forming a
pre-ceramic polymer pattern. Such methods, when employing elastomeric
stamps, are often referred to as "soft lithography." Regardless of the
type of tooling used, the mold must have sufficient resolution and good
release characteristics, which may be achieved either through selection
of the tooling material itself or subsequent treatment of the tooling
with reactive or inert species designed to enhance mold release.
Alternatively, the pre-ceramic can be molded in the absence of a
substrate.

[0037] The structures of the invention are what would be considered
epitaxial in that the substrate crystal structure plays a central role in
the formation of the structure of the film. The above-mentioned SiC films
are fabricated on a single crystal silicon substrate with a <100>
crystal orientation, the SiC forms a single crystal across the entire
substrate. Analysis of the structures has indicated that it taking on a
diamond cubic structure, known as β-SiC. Use of an amorphous
substrate did not show evidence of single crystal formation, but rather
the formation of polycrystalline domains on the order of 100 μm. The
sample sizes are as large as 2 cm, though this should not be interpreted
as a limitation of the approach.

[0038] The epitaxial process of the present invention has at least two
distinct advantages. First, it can be used with the above patterning
process. Also, it is able to form single crystal β-SiC without the
use of chemical vapor deposition. Further, the spin coating process
employed by the method of the invention offers a distinct cost advantage.
Any phase of SiC can be formed, depending on the substrate.
Alternatively, the material can be amorphous; the method is not exclusive
to a particular phase of SiC

Examples

[0039] For these experiments, electron beam lithography was used to form
arrays of nanowires as well as more complicated patterns. PMMA was spin
coated on silicon substrates to a thickness of 200 nm. The patterns were
created using a JEOL JSM 1401F field emission scanning electron
microscope (SEM) using the NPGS nanopatterning system. Following electron
beam exposure, the resist was developed in a 3:1 mixture of isopropyl
alcohol and methyl isobutyl ketone (MIBK). This mixture produces the
highest resolution patterns. Previous lithography tests with this SEM
have shown a minimum pattern linewidth of 50 nm in 200 nm PMMA.

[0040] Patterns that were tested consisted of arrays of lines and dots
with widths/diameters ranging from 50 nm to 500 nm. Several complex
patterns with numerous curved shapes, such as the University of
Massachusetts Lowell logo were also tried (FIG. 1). To assess the ability
to form large areas with a uniform SiC film, a pattern with a width of 20
μm and a length of 100 μm was made as well. The line and dot
patterns were measured with SEM (FIG. 2) as well as atomic force
microscopy (AFM) (FIG. 3). It was found that the process was capable of
forming structures with line widths at the minimum that the electron beam
lithography was capable of creating, 50 nm.

[0041] 500 μm thick <100> prime grade silicon wafers from
University Wafer were cleaned using an abbreviated form of the standard
`RCA` [a] cleaning procedure, which is incorporated herein in its
entirety. In particular, the first step was employed: Cleaning in a
mixture of NH4OH and H2O2. This was followed by successive
rinsing in microelectronics grade acetone, isopropyl alcohol, and
deionized water. These steps removed any organic contaminants present on
the wafer surface, and ensured good adhesion of the pre-ceramic polymer
following deposition and pyrolysis.

[0042] As the viscosity of pure SMP-10 was found to be too high to form
homogeneous films via spin coating, diluted SMP-10 was favored following
several experiments which showed higher concentration of solvent produces
better SiC upon pyrolysis. Several solvents were investigated for this
process as well, including xylenes, cyclohexane and methylcyclohexane.
Solutions based on xylenes were found to evaporate at an appropriate rate
for the formation of homogeneous films, but dissolved the PMMA to a large
extent. This favored the selection of cycloaliphatic hydrocarbons with
similar volatility to xylenes and the ability to dissolve SMP-10 without
dissolving PMMA or photoresists. In the finalized procedures, SMP-10 was
dissolved in cyclohexane at a 1:19 ratio of polymer to solvent by weight
(5 wt % SMP-10).

[0043] A 2 wt % solution (in anisole) of PMMA with a molecular weight of
950,000 g mol-1 was purchased from MicroChem. 500 μL of this PMMA
solution was applied to a silicon substrate using a positive displacement
microdispenser while the latter was spun at a speed of 600 rpm for 60
seconds using a CHEMAT KW-4A spin coater. The coated sample was then put
on a temperature-controlled hot plate (Tsurface=180° C.) for
15 minutes. Then, a pattern was created on the sample using the NPGS
nanopatterning system of a JEOL JSM-1401F field emission scanning
electron microscope (FE-SEM). Finally, the PMMA was developed in a 1:3
mixture of microelectronics grade methyl isobutyl ketone and isopropyl
alcohol.

[0044] Patterning has also been demonstrated using a UV based photoresist
(AZ1512 from AZ electronic materials). The AZ1512 was applied to the
substrate at 4000 RPM for 45 seconds. The freshly coated substrate was
baked at 90° C. for 45 seconds. A chrome plated exposure mask with
the desired pattern to be transferred was placed on top of the substrate.
The substrate was then placed under a Hg flood lamp with an
electronically timed shutter. Exposures lasted from 15-45 seconds
depending on the thickness of the resist used. Following exposure, the
substrate was placed in AZ 300 MIF Developer (AZ electronic materials)
for approximately 40 seconds, followed by a rinse in deionized water.

[0045] 500 μL of 5 wt % SMP-10 dissolved in cyclohexane was applied to
the substrate while the latter was spun at a speed of 4000 rpm for 60
seconds. After spin-coating, the homogeneous pre-ceramic polymer film
thus formed was ready for high temperature conversion to silicon carbide
directly in the case of PMMA resists. For the AZ1512 resist, the
substrate was placed in a vacuum oven and cured under a nitrogen
atmosphere for 3 hours at 120° C. Following curing, the resist was
removed via dissolution in acetone.

[0046] To avoid stress induced fracturing of the SiC film induced by rapid
increases in temperature, a series of stages of gradual heating are
required to yield intact structures. The pyrolysis step, following the
thermal profile detailed in Table 1, was carried out in a Lindberg 54233
tube furnace using an alumina tube with sealed, gas-ported end-caps under
flowing argon (60 mL min-1), as needed to prevent oxide formation.

[0047] We used photoconductivity tests with a visible light source to
demonstrate that the resulting SiC is of semiconducting quality (if it
were impure, the conductivity would not be so affected by illumination).
Illumination creates free electron hole pairs which contribute to
conductivity in the semiconductor. This effect would not be observed in
metals or insulators via the application of visible light as it requires
an interband transition to occur. FIG. 4 shows current vs. voltage curves
for undoped SiC films, with and without illumination with visible light.

[0048] We also demonstrated the fabrication of a silicon-silicon carbide
heterojunction diode. The pre-ceramic polymer in this case is doped with
nitrogen by ion implantation and is in contact with a phosphorous doped
p-type silicon substrate. Current vs. voltage curves across the junction
show a clear rectification effect, with current in the forward bias
direction three orders of magnitude larger than in reverse bias. The
orientation of the polarities correctly corresponds with the expected
anode and cathode terminals for the diode. Without doping of the SiC, no
rectification is observed and the current through the device is on the
order of nanoamps.

[0049] Current-voltage curves for Si--SiC heterojunction diode. The
devices shows a turn-on voltage of approximately 4 V, consistent with a
SiC device as shown in FIG. 5 (a silicon diode turns on around 0.5-0.7
V). FIG. 6 indicates that the diode shows excellent rectification with a
reverse bias current that is three orders of magnitude smaller than the
forward bias current.

[0050] Using this diode, we also demonstrated photoconductivity; with
light applied to the diode under bias, an increased current is found to
flow through the diode. This effect was demonstrated at elevated
temperatures. Testing has been conducted up to 170° C. In
contrast, silicon photosensors cease to work at temperatures above
100-150° C. The diode has also been shown to develop a
photovoltage when exposed to sunlight, indicating that it can also act as
a solar cell.

[0051] The teachings of all patents, including U.S. Pat. No. 5,234,537,
U.S. Pat. No. 3,421,956 and U.S. Pat. No. 6,034,001, published
applications and references cited herein are incorporated by reference in
their entirety. All references identified also are incorporated herein by
reference in their entirety.

[0052] While this invention has been particularly shown and described with
references to example embodiments thereof, it will be understood by those
skilled in the art that various changes in form and details may be made
therein without departing from the scope of the invention encompassed by
the appended claims.

Patent applications by Joel M. Therrien, Westford, MA US

Patent applications by UNIVERSITY OF MASSACHUSETTS

Patent applications in class Including variation in thickness

Patent applications in all subclasses Including variation in thickness